The golf balls are now under a rapid transition from the thread wound structure to the solid structure because most golfers favor the superior distance performance of solid balls.
The solid structure is initially typified by two-piece solid golf balls in which a solid core of rubber having excellent resilience is disposed at the center of the ball as the majority thereof and enclosed with a hard resin cover formed of ionomer resins or the like for providing protection against external damages.
Although the solid ball is good in distance, it undergoes a smaller deformation upon impact than the wound golf ball. The solid ball then gives a hard or unpleasant feel when hit. Since the smaller deformation corresponds to a smaller area of contact with the club face, the solid ball receives less spin and is thus less controllable on use of an iron club.
Many attempts were made to overcome these drawbacks, for example, by reducing the hardness of the solid core, placing a buffer layer between the core and the cover to form a three-layer structure, and using relatively flexible polyurethane as the outermost cover stock in the three-layer structure.
As a consequence, the feel and spin rate are improved to a substantially satisfactory level. Particularly when a polyurethane cover is used as the outermost layer in the three-layer structure, spin receptivity is improved over the ionomer resin covered balls. For the same reason of spin receptivity, however, the travel distance of the ball is not increased as expected when hit with a driver or a similar club intended for distance. Such shortage of travel distance can be compensated for by adjusting the hardnesses and gages of the core and two resin cover layers so that the spin receptivity of the ball when hit with a driver is reduced while the spin receptivity of the ball when hit with an iron club is kept unchanged. This technology has been established in a substantial sense.
By reducing or suppressing the spin the ball receives when hit with a driver, the distance characteristic of the solid golf ball is advantageously ensured. As a consequence of the reduced spin on driver shots, the difference in trajectory between pole hit (ball spins about axis on equatorial plane) and seam hit (ball spins about pole-to-pole axis) which has not been distinctly recognized by players becomes more significant.
The difference between pole hit and seam hit is described in detail. Most often, golf balls are molded using a mold composed of two mold halves which are removably joined to define a spherical cavity therein. Since the ball is molded in axisymmetry, the ball tends to have a lower sphericity about an axis existing on a plane circumscribed by a parting line (or equator) corresponding to the center of a line connecting the apexes (or poles) of the cavities of the mold halves. Owing to such a difference in sphericity, the prior art golf ball has a possibility that flight performance differs depending on the position where it is hit. In the golf play where the ball shall not be moved except in a special situation and shall be played as it lies according to the rules of golf, the variation in flight distance becomes a significant problem.